Hi Hans,
On Mon, Aug 21, 2017 at 06:36:29PM +0200, Hans Verkuil wrote:
On 08/21/2017 06:01 PM, Daniel Vetter wrote:
On Mon, Aug 21, 2017 at 5:52 PM, Brian Starkey <brian.star...@arm.com> wrote:
Hi all,
I couldn't find this topic talked about elsewhere, but apologies if
it's a duplicate - I'll be glad to be steered in the direction of a
thread.
We'd like to support DRM format modifiers in v4l2 in order to share
the description of different (mostly proprietary) buffer formats
between e.g. a v4l2 device and a DRM device.
DRM format modifiers are defined in include/uapi/drm/drm_fourcc.h and
are a vendor-namespaced 64-bit value used to describe various
vendor-specific buffer layouts. They are combined with a (DRM) FourCC
code to give a complete description of the data contained in a buffer.
The same modifier definition is used in the Khronos EGL extension
EGL_EXT_image_dma_buf_import_modifiers, and is supported in the
Wayland linux-dmabuf protocol.
This buffer information could of course be described in the
vendor-specific part of V4L2_PIX_FMT_*, but this would duplicate the
information already defined in drm_fourcc.h. Additionally, there
would be quite a format explosion where a device supports a dozen or
more formats, all of which can use one or more different
layouts/compression schemes.
So, I'm wondering if anyone has views on how/whether this could be
incorporated?
I spoke briefly about this to Laurent at LPC last year, and he
suggested v4l2_control as one approach.
I also wondered if could be added in v4l2_pix_format_mplane - looks
like there's 8 bytes left before it exceeds the 200 bytes, or could go
in the reserved portion of v4l2_plane_pix_format.
Thanks for any thoughts,
One problem is that the modifers sometimes reference the DRM fourcc
codes. v4l has a different (and incompatible set) of fourcc codes,
whereas all the protocols and specs (you can add DRI3.1 for Xorg to
that list btw) use both drm fourcc and drm modifiers.
This might or might not make this proposal unworkable, but it's
something I'd at least review carefully.
Otherwise I think it'd be great if we could have one namespace for all
modifiers, that's pretty much why we have them. Please also note that
for drm_fourcc.h we don't require an in-kernel user for a new modifier
since a bunch of them might need to be allocated just for
userspace-to-userspace buffer sharing (e.g. in EGL/vk). One example
for this would be compressed surfaces with fast-clearing, which is
planned for i915 (but current hw can't scan it out). And we really
want to have one namespace for everything.
Who sets these modifiers? Kernel or userspace? Or can it be set by both?
I assume any userspace code that sets/reads this is code specific for that
hardware?
I think normally the modifier would be set by userspace. However it
might not necessarily be device-specific code. In DRM the intention is
for userspace to query the set of modifiers which are supported, and
then use them without necessarily knowing exactly what they mean
(insofar as that is possible).
e.g. if I have two devices which support MODIFIER_FOO, I could attempt
to share a buffer between them which uses MODIFIER_FOO without
necessarily knowing exactly what it is/does.
I think Laurent's suggestion of using a 64 bit V4L2 control for this makes
the most sense.
Especially if you can assume that whoever sets this knows the hardware.
I think this only makes sense if you pass buffers from one HW device to another.
Because you cannot expect generic video capture code to be able to interpret
all the zillion different combinations of modifiers.
I don't quite follow this last bit. The control could report the set
of supported modifiers.
However, in DRM the API lets you get the supported formats for each
modifier as-well-as the modifier list itself. I'm not sure how exactly
to provide that in a control.
Thanks,
-Brian
Regards,
Hans
